92. Reverse Linked List II

O(n) time O(1) space
这道题的思路跟普通的reverse不一样！！
curr是固定的，永远指向初始的第m个结点，anchor也是固定的，永远是curr的前继结点
每次curr都是和它的succ交换，然后让next的下一个指向anchor的下一个结点，最后anchor的下一个指向那个succ结点

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseBetween(ListNode* head, int m, int n) {
        ListNode dummy_head(0), *anchor = &dummy_head;
        dummy_head.next = head;
        n -= m;
        while (--m > 0)
            anchor = anchor->next;
        auto curr = anchor->next;
        while (n-- > 0) {
            auto succ = curr->next;
            curr->next = succ->next;
            succ->next = anchor->next;
            anchor->next = succ;
        }
        return dummy_head.next;
    }
};